1. Field of the Invention
The invention relates to the processing of incident signals within a signal receiver. The invention therefore applies advantageously to a receiver which combines several components of multi-path signals mutually delayed by different time delays before reaching the receiver. Such a receiver is for example present in code division multiple access (CDMA system) wireless communication systems and is currently designated by those skilled in the art by the name of “Rake” receiver. The invention relates more particularly to the processing of the signals within a “Rake” receiver during a change of configuration of the fingers of the receiver.
2. Description of the Related Art
In a wireless communication system, a base station communicates with a plurality of remote terminals, such as cellular mobile telephones. Frequency division multiple access (FDMA) and time division multiple access (TDMA) are the conventional multiple access systems for delivering simultaneous services to a certain number of terminals. The basic idea underlying the FDMA and TDMA systems consists in dividing the available resource, respectively into several frequencies or into several time slots, such that several terminals can operate simultaneously without causing interference.
Telephones operating according to the GSM standard belong to the FDMA and TDMA systems in the sense that transmission and reception take place at different frequencies and also in different time slots.
Unlike these systems using frequency division or time division, CDMA systems (code division multiple access) enable the multiple users to share a common frequency and a common time channel by using a coded modulation. Amongst the CDMA systems we can cite the CDMA 2000 system, the WCDMA system (wide band CDMA) or the IS-95 standard.
In CDMA systems, as is well known to those skilled in the art, a scrambling code is associated with each base station and is used to distinguish one base station from another. In addition, an orthogonal code, known to those skilled in the art as the OVSF code, is allocated to each remote terminal (such as for example a cellular mobile telephone). All the OVSF codes are mutually orthogonal which distinguishes one channel from another.
Before transmitting a signal over the transmission channel to a remote terminal, the signal has been scrambled and spread by the base station using the scrambling code of the base station and the OVSF code of the channel.
In CDMA systems, we can still distinguish those that use a distinct frequency for transmission and reception (CDMA-FDD system) from those which use a common frequency for transmission and reception, but distinct time domains for transmission and reception (CDMA-TDD system).
The invention applies advantageously to communication systems of the CDMA type and more particularly to systems of the WCDMA type with terrestrial radio access (UTRA FDD/TDD).
The incident signal received by a mobile telephone for example comprises different versions delayed in time from the signal initially transmitted, which versions are the result of the multi-path transmission characteristics of the transmission environment between a base station and the telephone. And each path introduces a different delay.
The “Rake” receiver in a cellular mobile telephone operating in a CDMA communication system is used to carry out temporal alignment, descrambling, despreading, channel correction and combination of the delayed versions of the initial signals in order to deliver the information streams (symbols) contained in the initial signals.
A “Rake” receiver is generally formed of several “fingers”. Each finger is intended to demodulate a given path received at a given instant. The baseband demodulation consists essentially in a descrambling and a despreading. Next, the signal after despreading undergoes a channel correction, generally consisting of a phase correction and an amplitude correction. All these operations relating to demodulation are performed, within each finger, in several demodulation units respectively associated with various physical transmission channels.
To perform the combination of the delayed versions of the initial signals, a “Rake” receiver conventionally performs in theory the summation of the contributions delivered as output from all the fingers, in a static manner. Stated otherwise, the fingers are assumed to be active all the time and the configuration of the fingers is assumed not to change. Also, if fingers nevertheless have to be activated or deactivated, that is to say if the configuration of the fingers has to be modified, then this must be performed in theory at an appropriate moment in time.
Having said this, in practice, and in conventional “Rake” receivers, this static theoretical behavior cannot be guaranteed since the drive unit RMU (Rake Management Unit) will in general change the configuration of the fingers by adding or removing fingers in an arbitrary manner so as to optimize performance. However, when adding or removing these fingers in an arbitrary manner, a loss of symbols generally occurs, leading to errors.
FIG. 1 illustrates such a drawback.
More precisely, in this figure is considered an initial configuration in which three fingers, for example the fingers fng1, fng2 and fng3, are active. Next, at a given instant, the configuration of the fingers changes through the removal of the finger fng2 and the addition of a new finger, namely the finger fng4.
Represented in FIG. 1 are the relative instants of arrival of the outputs of the fingers (contribution of the symbols) and the result of the combination at the output of the means of combination.
The references Si at the bottom of the figure designate the symbols at the output of the means of combination while in the remainder of the figure the references Si designate the contributions of the various fingers in respect of these symbols.
Thus, in this example, before the arrival of the fifth symbol (contribution) S4 at the finger fng1, the finger fng2 is replaced with the finger fng4.
On account of the latency of the finger fng4, that is to say the time required for this finger to deliver a symbol contribution, two symbol contributions of this finger, namely the contributions of symbols S4 and S5, are missing. Consequently, and since in this example the finger fng4 is delayed by more than a symbol with respect to the other fingers, the symbols S3 and S4 emanating from the means of combination are formed only from the contributions stemming from the fingers fng1 and fng3.
Moreover, the symbol S5, and in fact all the following symbols, is composed of a mixture of contributions corresponding to different symbols (the symbol S5 emanating from the finger fng1, the symbol S5 emanating from the finger fng3 and the symbol S6 emanating from the finger fng4) and, consequently, this results in an error at the output of the means of combination.
Numerous situations may thus arise as a function in particular of the spreading factor, of the instant of modification of the configuration, of the finger considered, etc. with more or less serious consequences for the performance of the “Rake” receiver.
Accordingly, there exists a need for overcoming the disadvantages of the prior art as discussed above.